The invention relates to a hydraulic brake force booster for a vehicle braking system having a control valve which is actuated by a brake-force exerted by a brake pedal. The distance of brake pedal movement is determined by a travel-imparting spring. The braking system monitors hydraulic pressure in connections between a supply source and a main cylinder, which receives a piston of the brake force booster, and between the main cylinder and a relief source. A cutoff assembly is provided to release the limits imposed on the travel-imparting spring in response to failure in the supply pressure. The cutoff assembly also includes a barrier piston which is subject to an arresting pressure. A brake force booster of this kind is known (German Offenlegungsschrift No. 26 09 905 corresponding U.S. Pat. No. 4,094,554). In brake force boosters of this kind, a problem exists wherein the cutoff assembly is actuated only very seldom--in fact, only if the supply pressure fails. The cutoff assembly can become sluggish as a result and then cannot be capable of functioning in an emergency.
In the case of integrated hydraulic brake boosters having a control valve located parallel to and beside the main cylinder or cylinders, the problem is still more complex. The push rod which actuates the control valve must be supported, once the control valve has switched over to the braking direction, on a stop to provide the driver with a sense of brake pressure exerted. This is accomplished with the travel-imparting spring which is disposed between the control valve and the pedal. If this stop were not present in the apparatus, then the pedal force would not increase any further after the brake pressure exertion point of the control is reached; it would remain virtually constant, until finally the pedal push rod reached the piston in the main cylinder. Only then, if the brake pedal is depressed still further, would there be a further increase in the counter-acting force. This is unfavorable in human-engineering terms.
Accordingly, it has been proposed to use an auxiliary piston dependent on supply pressure. In the event the supply pressure fails, the auxiliary piston pushes open a check valve which has previously kept a chamber closed, the movable wall of which is formed by a barrier piston which supports the control valve. This solution has the disadvantage, however, that the auxiliary piston comes into action very infrequently. There is thus the danger that if a failure of supply pressure already exists, a previously unknown defect in the device could prevent optimal functioning of the brake force booster.